1. Field of the Invention
This invention relates to a magneto-optical medium adapted to reproduce information, utilizing displacement of domain walls by temperature gradient and also to a reproducing method for such a medium.
2. Related Background Art
Various magnetic recording mediums have been marketed as rewritable recording mediums. Particularly, magneto-optical recording mediums to be used for writing magnetic domains to record information there by means of thermal energy of a semiconductor laser and reading the stored information by means of magneto-optical effects are expected to develop into large capacity portable mediums capable of densely storing information in the future. In recent years, there has been an ever-increasing demand for raising the recording density and hence the storage capacity of such magnetic recording mediums in order to accommodate the current trend of digitizing moving images.
Generally, the linear recording density of an optical recording medium largely depends on the laser wavelength of the reproducing optical system and the numerical aperture NA of the objective lens. In other words, the diameter of the beam waist is defined by the laser wavelength λ of the reproducing optical system and the numerical aperture NA of the objective lens. Then, the spatial frequency of recording pits that can be used for signal reproduction is 2NA/λ at most. Therefore, it is necessary to either use a short laser wavelength for the reproducing optical system or increase the numerical aperture of the objective lens for realizing a high recording density in a conventional optical disk. However, it is not easy to reduce the laser wavelength from the viewpoint of operating efficiency of devices and the problem of generation of heat. Additionally, as the numerical aperture of the objective lens is increased, more and more rigorous requirements are imposed on the mechanical precision of devices because of a shallow focal depth and other reasons.
In view of these problems, various so-called super-resolution technologies providing novel configurations for signal reproduction from recording mediums have been developed to improve the recording density without changing the laser wavelength and the numerical aperture.
For example, Japanese Patent Application Laid-Open No. 3-93058 proposes a signal reproduction method using a multilayer film having a memory layer where signals are recorded and a readout layer that are magnetically coupled with each other and transferring the recorded signals from the memory layer by firstly aligning the directions of magnetization of the readout layer and subsequently heating the readout layer by means of laser beam irradiation so that the signals may be transferred onto the heated region of the readout layer, while the transferred signals are simultaneously read out. This method can reduce inter-symbol interferences during the signal reproducing operation and reproduce signals with a spatial frequency greater than 2NA/λ because the region from which the recorded signals are retrieved by heating it with the laser beam until it gets to the signal transfer temperature can be confined to an area smaller than the spot diameter of the signal reproducing laser beam.
However, the above proposed signal reproduction method has a drawback that the region that is effectively used for signal detection and signal retrieval is smaller than the spot diameter of the signal reproducing laser beam and hence the reproduced signal shows only a small amplitude and a small output level. In other words, the region that is effectively used for signal detection cannot be reduced excessively relative to the spot diameter. Thus, the proposed method cannot remarkably raise the recording density relative to the recording density is theoretically limited by the diffraction of the optical system.
In an attempt for dissolving the above problem, Japanese Patent Application Laid-Open No. 6-290496 discloses a method that can retrieve signals recorded at a high density exceeding the resolving power of the optical system without reducing the amplitude of the reproduced signal by moving the domain walls located along the boundary section of each recorded mark (magnetic domain) to the high temperature side along the temperature gradient produced in the recording medium.
Now, this signal reproducing method will be described below in greater detail.
FIGS. 8A through 8C of the accompanying drawings schematically illustrate a magneto-optical recording medium and the information reproducing method to be used for such a magneto-optical recording medium as disclosed in the above cited patent document. FIG. 8A is a schematic cross sectional view of the magneto-optical recording medium showing its configuration and the magnetized condition of an area irradiated with a signal reproducing light beam and FIG. 8B is a graph illustrating the temperature distribution produced in the magneto-optical recording medium when irradiated with the light beam, whereas FIG. 8C is a graph illustrating the distribution of the domain wall energy density σ of the domain wall displacement layer relative to the temperature distribution of FIG. 8B.
As shown in FIG. 8A, the magnetic layer of the magneto-optical recording medium has a multilayer structure formed by sequentially laying a magnetic layer 111 that is a domain wall displacement layer, another magnetic layer 112 that is a switching layer and still another magnetic layer 113 that is a memory layer, of which the magnetic layer 111 is arranged at the side to be irradiated with a signal reproducing light beam. In FIG. 8A, arrows 114 in the layers indicate the directions of atomic spin. Each domain wall 115 is formed along the boundary section of two regions where the respective directions 114 of atomic spin are inverted relative to each other.
In FIG. 8A, arrow 118 indicates the direction in which the recording medium is moved. The light beam spot 116 moves along the information track of the recording medium as the medium is displaced in the moving direction 118. As shown in FIG. 8B, the temperature T of the area irradiated with the light beam spot 116 rises from the front of the spot as viewed in the moving direction of the beam to produce a temperature distribution where the temperature gets to a peak at position Xc. Also note that the medium gets to temperature Ts that is close to the Curie temperature of the magnetic layer 112 at position Xa.
As shown in FIG. 8C, the distribution of the domain wall energy density σ of the magnetic layer 111 reaches the lowest level at a position near the temperature peak located near the tail end of the light beam spot 116 and gradually increases toward the front end of the spot. When the domain wall energy density σ shows a gradient along the variable position X, the domain wall of each of the layers at the position X is subjected to force F that is defined by equation (1) below.F=∂σ/∂X  (1) 
The force F is exerted so as to move the domain walls toward the lower domain wall energy area. Since the domain wall coercivity of the magnetic layer 111 is small and the domain wall is apt to be displaced to a large extent, the domain wall 115 will be easily moved by the force F when the magnetic layer is of a single layer structure. However, the temperature of the medium is lower than Ts and the magnetic layer 111 is coupled by exchange coupling to the magnetic layer 113 showing large domain wall coercivity in the region located in front of the position Xa relative to the spot so that the domain wall 115 is not displaced but rigidly held to a position corresponding to the domain wall in the magnetic layer 113 having large coercivity.
With this magneto-optical recording medium, as it is moved in the proper moving direction 118 and the domain wall 115 of the magnetic layer 111 gets to position Xa, the temperature of the medium at the domain wall 115 rises to Ts that is close to the Curie temperature of the magnetic layer 112 to break the exchange coupling between the magnetic layers 111 and 113. As a result, the domain wall 115 of the magnetic layer 111 is instantaneously displaced toward a region showing higher temperature and smaller domain wall energy density as indicated by broken-lined arrow 117. As the domain wall 115 passes below the light beam spot 116, the atomic spins of the magnetic layer 111 are forced to point a same direction.
Each time the domain wall comes to the position Xa as a result of the movement of the medium, it instantaneously passes below the light beam spot 16 to expand the recording domain to stretch from the position Xa to the position Xc and force the atomic spins of the magnetic layer 111 to point a same direction. Then, the retrieved signal constantly shows the largest possible amplitude without being restricted by the length between the domain walls where the signal is recorded (or the length of the recorded mark) and is completely freed from the problem of waveform interference caused by the optical limit of diffraction and other problems.
However, with the above described signal reproduction method according to Japanese Patent Application Laid-Open No. 6-290496, the force F necessary for causing the displacement of the domain wall is not particularly large at temperature close to Ts as shown in FIG. 8C. Thus, the starting point of displacement of the domain wall can fluctuate to give rise to large jitters to the retrieved signal so that the quality of the retrieved signal can be degraded.